Negatory on that. The downsized models are a bunch more fuel efficient and shouldn't be compared. OP's mileage sounds typical. The Rochester 2gc on the 400 is a pig, ditch it for a Quadrajet or other "spread bore" carb. Also worth investigating would be an "RV" cam for the engine. These were a very mild grind designed for low end torque which is what it takes to get one of these barges going several hundred times a day in city traffic. I put one in a big Dodge wagon years ago and it was an improvement in every way. The engine could be made to smoothly idle down to about 400 rpm, low enough to cause the oil pressure light to flicker. It gave the engine more grunt down low which meant less throttle opening for the same acceleration.